Deformation Behavior of Intermetallics: Models and Experiments

A sufficiently general thermally activated mechanism for extension of dislocations in some preferential direction was proposed. This mechanism comprises a necessary step of dislocation transformations that lead to blocking. The reasons for blocking of different types of dislocations in different materials are diverse. A new concept was developed concerning the possibility of thermally activated blocking of superdislocations in the absence of external stresses. Some experiments with single crystals of Ni3(Al, Nb) were performed. They included no-load heating after preliminary lowor high-temperature deformation. It was found that the initial dislocation structure, which included curvilinear dislocations, transformed to a set of long rectilinear blocked superdislocations after no-load heating. The experimental results confirmed theoretical assumptions on the possibility of thermally activated transformations of superdislocations to indestructible barriers in the absence of external stresses. Introduction High-temperature intermetallics like Ni3Al and TiAl possess a number of unusual properties (1, 2) including an anomalous temperature dependence of the yield stress σy(T). This behavior is mainly due to specific features of their dislocation structure. The ability of dislocations in intermetallics for self-blocking distinguishes them from dislocations in other materials. The problem at issue is thermally activated transformations of dislocations from the glissile form to the blocked configuration when transformations do not involve other dislocations (3). A Kear-Wilsdorf barrier is the blocked form of superdislocations with the Burgers vector <101> in Ni3Al and TiAl. It is known that the formation of a Kear-Wilsdorf barrier is stimulated by the anisotropy of the antiphase boundary (APB) energy, by which ς ς ′ < . Here and ς ς ′ denote energies per unit area of the APB in the octahedral and cube planes respectively. Either a Kear-Wilsdorf barrier or a "roof"-type configuration represents possible blocked forms of superdislocations with the Burgers vector 1/2<112> in TiAl. Blocking of a single dislocation with the Burgers vector 1/2<110> in TiAl is explained, in our opinion (4), by its localization in deep Peierls valleys extended in <110> directions. This mechanism is somewhat analogous to the dislocation blocking mechanism in semiconductors. However, we deal with deep valleys for single dislocations in TiAl and for partial dislocations in semiconductors (5, 6). A sufficiently general approach was developed earlier for description of the plastic deformation process (4). Many features of the deformation behavior of intermetallics could be explained in terms of this approach. However, some questions still remained unclear. By this study we tried to answer some of them, specifically: • Is blocking of dislocations possible in the absence of external stresses? If yes, in what conditions? • Can barriers be destructed in this case?